As is well known, internal combustion engines generate vibrations during operation. These vibrations get transmitted to the vehicle or device to which they are mounted. Mounts are typically mounted between the engine and the vehicle or device to actively or passively reduce the transmission of the vibrations thereto. The effectiveness of the mounts is related to both their type and their location amongst other factors. Mounts are also typically more effective over certain ranges of speed of the engine. Typically, in marine outboard engines, a soft mount is desired for operation at idle and low engine speed, while a stiffer mount is desired for operation at higher engine speeds.
FIG. 2 illustrates a side view of a drive unit mount 10 used in marine outboard engines which at least partially addresses the above issue. The drive unit mount 10 includes a central rigid member 12, a top rigid member 14, a bottom rigid member 16, a top resilient member 18 and a bottom resilient member 20. As can be seen, the central rigid member 12 is wedge-shaped. The top and bottom rigid members 14, 16 are also wedge-shaped and arranged, as shown in FIG. 2, above and below the central rigid member 12. The top resilient member 18 is disposed in the angled space defined between the flat top surface of the central rigid member 12 and the flat bottom surface of the top rigid member 14. The bottom resilient member 20 is disposed in the angled space defined between the flat bottom surface of the central rigid member 12 and the flat top surface of the bottom rigid member 16.
A front 22 of the central rigid member 12 is connected to a bracket (not shown) used to connect a drive unit (not shown) of the marine outboard engine to the stern of a boat. The bracket is disposed between the stern of the boat and the drive unit. The drive unit is connected to the drive unit mount 10 by the top and bottom rigid members 14, 16. When the engine of the drive unit is in operation and a transmission thereof is in a position to engage a propeller in order to propel the marine outboard engine and therefore the boat forwardly, the drive unit applies a force, indicated by arrows 24, on the drive unit mount 10. The magnitude of this force is a function of the engine speed. This force causes the top and bottom rigid members 14, 16 to move slightly forwardly relative to the central rigid member 12. As a result, the top resilient member 18 is compressed between the top rigid member 14 and the central rigid member 12 and the bottom resilient member 20 is compressed between the bottom rigid member 16 and the central rigid member 12. As the force being applied increases, the more the resilient members 18, 20 are compressed. As the degree of compression of the resilient members 18, 20 increases, the stiffer the resilient members 18, 20 become. Therefore, the drive unit mount 10 has a variable stiffness depending on engine speed.
Although the above drive unit mount 10 addresses the issue of providing variable stiffness, its stiffness only varies as a result of the force indicated by the arrows 24 which is generated by the propeller. The drive mount unit 10 has a constant and low stiffness laterally. As such, the stiffness of the drive mount 10 is not ideally suited for absorbing vibrations when high lateral forces are applied to the drive mount 10 when the marine outboard engine is turned to steer the boat.
Thus, although the drive unit mount illustrated in FIG. 2 provides adequate vibration damping under many operating conditions, there is a need for a drive unit mount for a marine outboard engine that also absorbs vibrations over a range of lateral forces applied to the drive unit, as a result of steering of the drive unit for example.